1. Field of the Invention
The present invention relates to a method and apparatus for acoustic detection and location of defects in structures and/or ice on structures, and more particularly, to the non-destructive acoustic testing and evaluation of materials and mechanical structures utilizing an ultrasonic probing signal and low frequency vibration signal to identify and locate ice or integrity-reducing flaws such as cracks, fractures, delamination, unbonding, etc.
2. Related Art
Conventional active acoustic methods of ice detection and of non-destructive testing (NDT) are based on the principles of linear acoustics. These include effects of reflection, scattering, transmission, and absorption of probe acoustic energy. The presence of ice or a defect leads to phase and/or amplitude variation of received signals while the frequencies of the received signals are the same as the emitted probe signals.
The principal difference between the modulation technique of the present invention and linear acoustic NDT techniques is that the modulation technique correlates the presence and characteristics of a defect or of a material such as ice, with acoustic signals whose frequencies differ from the frequencies of the emitted probe signals. These signals with different frequencies are an outcome of a modulation transformation of the probe acoustic energy by a defect.
The modulation NDT methods have a number of advantages as compared with the linear acoustic techniques. Among them are high sensitivity and applicability to highly non-homogeneous and/or geometrically complex structures, such as composites, engine components, etc.
Previous efforts at NDT techniques include:
Perchersky, U.S. Pat. No. 5,520,052, discloses a method and apparatus for determining material structural integrity by combining laser vibrometry with damping analysis techniques to determine the damping loss factor of a material. The method comprises the steps of vibrating an area to be tested over a known frequency range and measuring vibrational force and velocity as a function of time over the known frequency range. Using known vibrational analysis, a plot of the drive point mobility of the material over the pre-selected frequency range is generated from the vibrational force and velocity measurements. Once computed, the damping loss factor can be compared with a reference stamping loss factor to evaluate the structural integrity of the material.
Larsen, U.S. Pat. No. 5,170,666 discloses a nondestructive evaluation of composite materials using acoustic emissions stimulated by absorbed microwave/radiofrequency energy. A specimen is exposed to pulsed radio frequency energy to produce an elastic wave that propagates on the surface of the specimen. The wave is detected by a piezo-electric or electro-optic displacement mode transducer which produces a signal corresponding to the wave. The signal is analyzed by a processor and classified.
Tsuboi, U.S. Pat. No. 5,214,960 discloses a method and apparatus for detecting defects in an object by vibrating the object in a plurality of positions. While the test object is vibrating, signals indicative of the vibration of the test object are detected and a signal indicative of a natural vibration of the test object is produced, and a signal indicative of a defect-induced vibration of the test object is produced. The signal indicative of the natural vibration and the signal indicative of the defect-induced vibration are compared to determine whether there is a defect in the test object.
Wajid, et al., U.S. Pat. No. 5,528,924 discloses an acoustic tool for analysis of a gaseous substance, specifically a refrigerant gas, to determine whether the sample contains significant contaminants. The refrigerant is tested by introducing a vapor sample into a resonant chamber which is formed to produced two distinct resonances, the resonator having first and second necks connecting first and second volumes. A frequency generator produces a sweep of frequencies in a band and then includes the two resonances and the sweep is applied to a transducer in one of the volumes. Another transducer responsive to vibrations produces an output signal that varies in response to the amplitude of the vibrations in the chamber. A digital circuit responsive to the frequency generator and the second transducer output determines the center frequencies for the first and second resonances and determines the frequency width of these resonances to determine quality or sharpness factors for the two resonances. Then the center frequencies and sharpness factors are compared with storage data and a determination as to the nature and extent of contaminants is made.
Rhodes, et al., U.S. Pat. No. 5,425,272 discloses the use of relative resonant frequency shifts to detect cracks. At least two prominent resonant frequencies of an object are sensed and the frequency difference is measured. The ratio of the frequency difference to one of the prominent resonant frequencies is determined and compared to predetermined criteria. Resonant frequency dependent upon dimensions will shift very little while resonant frequency dependent upon stiffness will shift a relatively large amount when an object has a crack.
Dixon, et al., U.S. Pat. No. 5,355,731 discloses a method for grading production quantities of spherical objects. A resonant ultrasound spectroscopy (RUS) spectrum is generated from a spherical object. Sphere parameter values for the spherical object are determined from first components of the RUS spectrum. An asphericity value of the spherical object is determined from second components of the RUS spectrum and the spherical parameter values. The asphericity value is then compared with predetermined values to grade the spherical product.
Jones, U.S. Pat. No. 5,284,058 discloses a method for measuring complex shear or Young's modulus of a polymeric material wherein first and second beams of preselected lengths and different thickness are disposed in parallel spaced relationship firmly held at the ends thereof and first and second spaced gripping members are attached along the beams, a specimen of polymeric material is disposed between confronting surfaces of the gripping members, a time varying force is applied to one beam, the time varying displacements of the beams are measured, and the modulus of the polymeric material is calculated from the measurements.
Tsuboi, U.S. Pat. No. 5,179,860 discloses a defect detecting method which includes the steps of vibrating the object, picking up the vibration, and detecting that a spectrum of the characteristic vibration of the object to be measured is separated into two portions. The method can also be used to detect cracks by vibrating an object, picking up the vibration, and detecting that an odd order spectrum of the characteristic vibration of the object to be measured is separated into two portions. A non-through defect can be determined in the same way by detecting that an even order spectrum of the characteristic vibration of the object to be measured is separated into two portions.
Tsuboi, U.S. Pat. No. 5,144,838 discloses a defect detecting method which includes the steps of vibrating the object, picking up the vibration, and detecting that a spectrum of the characteristic vibration of the object to be measured is separated into two portions. The method can also be used to detect cracks by vibrating an object, picking up the vibration, and detecting that an odd order spectrum of the characteristic vibration of the object to be measured is separated into two portions. A non-through defect can be determined in the same way by detecting that an even order spectrum of the characteristic vibration of the object to be measured is separated into two portions.
Clark, Jr. et al., U.S. Pat. No. 4,944,185 discloses a method for nondestructively inspecting the integrity of a material by tagging the material, applying the material, activating the tagged particles to cause an inherent structural resonance in the tagged material, monitoring and measuring the structural resonance of the material with a probe, and relating the structure resonance of the material to the structural integrity of the material. The invention has particular application to a material such as an adhesive material.
Slettemoen, U.S. Pat. No. 4,689,993 discloses a method and apparatus for measuring and mapping vibrations wherein one or more local sensors and a measuring means make local registrations and frequency decompositions of the vibrations of an oscillating object. The same sensors and measuring means can also be used with an image-forming unit and an associated measuring means for local and image-forming recording of the vibrations of an oscillating object.
Chamuel, U.S. Pat. No. 4,461,178 discloses a method for ice detection wherein a flexural wave and a compressional wave is propagated through a structure. Ice impacts the propagation speed of the flexural wave, as do others, i.e. temperature, etc. These other factors are taken into account by the use of the compressional wave which provides a baseline for the structure at the particular conditions encountered.
Yost, U.S. Pat. No. 5,736,642 disclose a method of nonlinear ultrasonic scanning to detect material defects wherein first and second frequencies are propogated and combination frequencies result, i.e. a sum wave or a difference wave (f.sub.1.+-.f.sub.2).
Additionally, there have been previous efforts at producing methods for detection of ice.
Conventional methods of ice detection utilize various types of energy: cross-polarized coherent light (U.S. Pat. No. 5,650,610) laser beam (U.S. Pat. No. 5,823,474), fiber optic sensors (U.S. Pat. No. 5,748,091), electrical conductivity sensors (U.S. Pat. No. 5,621,400), piezoelectric film sensor (U.S. Pat. No. 5,206,806), and ultrasonic sensors (U.S. Pat. Nos. 4,461,178 and 5,456,114). Ultrasonic sensing devices are based on the principles of linear acoustics. These include effects of reflection, scattering, transmission, absorption of probe acoustic energy. The presence of ice coating leads to phase and/or amplitude variation of received signals while the frequencies of the received signals are the same as of emitted probe signals.
None of these previous efforts, taken either alone or in combination teach or suggest all of the elements, nor all of the benefits and utility of the present invention.